Various natural and recombinant proteins have medical and pharmaceutical utility. Once they have been purified, separated, and formulated, they can be parenterally administered for various therapeutic indications. However, parenterally administered proteins may be immunogenic, may be relatively water insoluble, and may have a short pharmacological half life. Consequently, it can be difficult to achieve therapeutically useful blood levels of the proteins in patients.
These problems may be overcome by conjugating the proteins to polymers such as polyethylene glycol. Davis et al., U.S. Pat. No. 4,179,337 disclose conjugating polyethylene glycol (PEG) to proteins such as enzymes and insulin in order to result in conjugates where the protein would be less immunogenic and would retain a substantial proportion of its physiological activity. Nakagawa, et al. disclose conjugating PEG to islet-activating protein to reduce its side-effects and immunogenicity. Veronese et al., Applied Biochem. and Biotech, 11:141-152 (1985) disclose activating polyethylene glycols with phenyl chloroformates to modify a ribonuclease and a superoxide dimutase. Katre et al. U.S. Pat. Nos. 4,766,106 and 4,917,888 also disclose solubilizing proteins by polymer conjugation. PEG and other polymers are conjugated to recombinant proteins to reduce immunogenicity and increase half-life. See Nitecki, et al., U.S. Pat. No. 4,902,502, Enzon, Inc., International Application No. PCT/US90/03133, Nishimura et al., European Patent Application 154,316 and Tomasi, International Application Number PCT/US85/02572.
Previous methods of forming PEG/protein conjugates and the conjugates which result from said methods present several problems. Among these problems is that certain methods of forming these protein-PEG conjugates may inactivate the protein so that the resulting conjugates may have poor biological activity. In addition, certain linkers utilized in forming these PEG-protein conjugates may be susceptible to in vivo hydrolytic cleavage. When such cleavage occurs after administration, these conjugates lose the beneficial properties provided by PEG.